The production of containers of thermoplastic material is generally relatively complex, and the container bottom is one of the parts requiring the most attention. It is the container bottom which supports the weight of the column of liquid contained in the container, and if it has insufficient mechanical strength, it loses its shape and the container can no longer rest in a stable manner on a flat support.
The container bottom is therefore the part of the container which it is particularly important to blow mold or stretch-blow mold into the correct shape, but it is a difficult part to shape because the thickness of the thermoplastic material in the bottom of the preform is substantially greater than elsewhere, which results in slower stretching and more difficulty with properly forming this container bottom.
Improper distribution of material at the container bottom can affect the stability of the container. Such improper distribution can occur in the central portion of the bottom and/or in the peripheral wall, which could modify the position and/or slope of the container base, and the container may no longer rest in a stable manner on a flat support.
To improve its mechanical strength, the bottom conventionally has a complex or highly complex shape, with many raised or sunken areas which when properly formed are supposed to provide the desired mechanical strength.
At any location on the bottom, the mechanical strength must be sufficient to prevent the bottom collapsing from the weight of the liquid and the internal pressure (as this type of container is often used to contain carbonated liquids).
Document US 2004/0251258, for example, shows several container models which have a relatively complex bottom; this bottom comprises several distinct regions of different diameters with radii of curvature that also change from one zone to another, forming a tiered bottom. The diameter of the supporting base of the bottle is relatively modest compared to the diameter of the container body, and above the part encircling this supporting base is a second level which is bowl-shaped and reinforced with vertical ribs to improve its strength.
The complex form of the containers, and particularly their bottom, requires accurate and careful control of the blowing conditions, particularly the blowing pressure, if properly made containers are to result.
The forming and shaping of the material in the bottom region are currently obtained by blowing under high pressure (typically on the order of 40×105 Pa), which allows combining such forming and shaping at high production speeds of several thousand containers per hour.
However, producing a blown fluid (generally air) at such a high pressure is very costly in terms of the materials and electricity used, and manufacturers want to cut costs at this station by using lower pressures and therefore less costly materials. In addition, it is difficult to conceive of an increase in the production speeds under current manufacturing conditions.
One should also stress the sometimes complex path followed by the material as it passes from its initial shape (preform) to that of the mold wall which corresponds to an impression of the final container; said impression can force this material, especially at the bottom of said container, into a path with changes in direction or even points of inflection, and these changes in direction can correspond to changes in angle of up to 90° or more.
This, together with the significant thickness of the material in this bottom region of the preform, can lead to insufficient coating of the material on the wall of the mold cavity and improper cooling of the material against the mold; a reduction in the quality of all or part of the container bottom may result. In addition, this travel of the material from the preform to the mold wall requires a certain amount of time.
Manufacturers of containers of thermoplastic material are always seeking to improve the economic conditions of production, by attempting to produce containers requiring a smaller amount of thermoplastic material, at a substantially lower blowing pressure if possible (likely to result in insufficient coating of the mold cavity) and especially at increasingly greater production speeds (requiring faster displacement of the material during molding).
The most difficult parts to blow mold are the parts which are at the greatest distance from the central axis of the mold bottom.
This difficulty is even more pronounced when there are regions of the mold bottom where the blown material is likely to accumulate; such accumulation can prevent the material from flowing freely and coating the entire mold bottom.
Such is the case when a rib is added to the mold bottom to reinforce the radial rigidity of the container bottom. This solution results in difficulties in coating the rib with blown material and can block the flow of blown material.
Such problems can occur with containers such as those described in the documents FR 2 892 048 and GB 2 041 286. In addition, in the case of document GB 2 041 286, it should be noted that the presence of a flange destabilizes the bottles during handling and particularly during conveyance; the containers have a tendency to jam together and climb one other, which adversely impacts their stability.
Therefore no solutions exist in the prior art which allow proper molding of a container bottom with reinforced radial rigidity while facilitating such molding by blowing or stretch-blowing an appropriate preform. The invention proposes a solution to these problems.